In the future it is expected that, for stationary applications such as, for example, wind turbine systems, emergency current units or island networks, as well as in vehicles such as, for example, hybrid vehicles or pure electric vehicles, battery systems will increasingly be used on which stringent requirements are made in terms of useful energy content, charging/discharging efficiency and reliability.
In order to be able to meet with the requirements with respect to available energy content, maximum power and overall voltage, many individual battery cells are connected in series and partially additionally in parallel. For example, in hybrid vehicles and electric vehicles, battery packs are used comprising Li ions or NiMH technology, which battery packs are composed of a large number of electrochemical cells connected in series.
Such a large number of battery cells which are connected in series entail a number of problems. For safety reasons and in order to achieve sufficient accuracy in the measurement of the voltage, the cell voltages of the individual battery cells must be measured individually and checked for compliance with upper limits and lower limits. Because of the series connection of the battery cells, the same current flows through all the battery cells, i.e. the quantity of the charge which is extracted during discharging and input during charging is also identical for all the battery cells. As a result, the cell with the lowest capacity limits the overall charge of the pack. Since as a rule there is a direct relationship between the loss of capacity and the increase in internal resistance for electrochemical cells, the cell with the lowest capacity generally also has the lowest power. If the capacity of a battery cell therefore differs from that of another, for example due to aging, the battery cells with a relatively high capacity can only be charged as far as the battery cell with the lowest capacity. Furthermore, the defect in an individual battery cell causes the entire battery to fail because the current can no longer flow through the defective battery cell and therefore through the battery.
A measure of the quantity of energy stored in a battery cell is what is referred to as the state of charge (SOC). It is to be noted here that the initial states of charge of the battery cells when they are assembled to form a battery will never be precisely the same. Furthermore, owing to a certain product variation during manufacture, the battery cells always differ slightly in their parameters and therefore also in their reaction of the state of charge to a current which is impressed from the outside. These differences can increase further due to aging of the battery cells.
It is known to use a battery management system to monitor the states of charge of a battery. In addition to the monitoring of safety, the longest possible service life of the battery is to be ensured and it is to be ensured that the states of charge of the individual cells are matched to one another. This is done by suitable cell balancing. Cell balancing or balancing of the states of charge is generally performed resistively. For this purpose, a resistor and a switching element are provided for each cell in order to be able to discharge individual cells in a targeted fashion. DE 10 2006 022 394 A1 discloses a device for balancing the charge of an energy source with a plurality of cells, in which the cells are connected to a discharging unit which has the purpose of balancing charges and at least partially discharges the cells. However, according to the prior art, it is also possible to carry out the cell balancing capacitively, i.e. with connected capacitors, or inductively, that is to say by means of connected inductors. In these two cases, energy can be exchanged between the cells with a limited efficiency while in the case of resistive cell balancing energy can only be converted into heat, and is therefore lost.
For hybrid vehicles, a high performance level is advantageous at any time, both in the charging direction for the purpose of recuperation and in the discharging direction for the purpose of acceleration. It is known that the maximum permitted charging power decreases as the state of charge rises, while the maximum permitted discharging power increases. For this reason, according to the prior art for hybrid vehicles, it would be desirable to operate a battery pack for hybrid vehicles in a state of charge of 50% in order to constantly ensure a high power. However, as a rule, in practice an operating window is used, for example between 40% and 60% of the state of charge. For what are referred to as “plug-in hybrids”, the operating window is correspondingly larger, for example 10% to 90% of the state of charge.
In contrast, in pure electric vehicles the energy content is the most important criterion. In contrast to hybrid vehicles, this relates less to short-term acceleration but instead more to ensuring the operational reliability of the vehicle over the longest possible time period, and of making available the largest possible amount of energy at any time.
A prior art balancing strategy attempts to achieve a continuously equal state of charge of all the cells. For this purpose, as a rule, balancing to an identical quiescent voltage of all the cells is performed. In the case of new cells with a virtually identical capacity, this strategy is justified. However, in the case of cells with differing capacities, such as occur due to production variations and aging, this balancing strategy leads to unnecessary energy losses as a result of the balancing.
FIG. 1 shows by way of example six electro-chemical cells with different charging capacities owing to aging, which cells have been adjusted to an individual state of charge of 50% according to such a balancing method according to the prior art. If the battery pack is then discharged until the cell with the lowest capacity, which is cell 5 in the example, is almost completely discharged, all the other cells have relatively high states of charge viewed individually. When the balancing is continued according to the above strategy, they are therefore discharged until they have also reached the individual state of charge of cell 5. However, this means that energy has to be unnecessarily destroyed in all the cells apart from the cell with the lowest capacity.